The invention relates to manufacturing of anionic polycaprolactam with a low content of monomer and stable degree of polymerization of the molten polymer.
Any polymerization product of caprolactam contains, after completed polymerization, the equilibrium amounts of polymer and monomer. This composition depends on the polymerization temperature and amounts, e.g. at 250.degree. C, to about 89% of polymer and 11% of water-extractable materials (further called extractables) which consist of monomer and oligomers. The equilibrium is shifted to the polymer with decreasing polymerization temperature. Thus, the polymer fraction increases to 95% at 200.degree. C. High content of extractables in the polymerization product unfavourably affects the properties of the final article from polycaprolactam. The polymerization product required by customers has a minimum content of extractables below 3%. The polymerization temperature corresponding to such equilibrium content of polymer and monomer in the product is about 170.degree. C. If the common concentrations of catalyst components, e.g. N-acylcaprolactam and sodium caprolactamate, are used, the polymerization mixture should be heated for a rather long period (more than 30 minutes) during which the polymer solidifies and cannot be conveyed in a simple way. This is unsuitable for the continuous manufacturing. Shorter polymerization periods may be achieved with increased concentration of catalyst components; however, this results in a lower stability of the degree of polymerization. Both requirements, i.e. low extractables and stability of the degree of polymerization, can be met in the adiabatic anionic polymerization of caprolactam. However, this discontinuous method is economically inconvenient in comparison to the continuous production and, in addition to this, large castings need to be expensively disintegrated in costly equipment.
This is why new routes have been sought for preparation of the polymer with reduced content of extractables. For example, the polymerization is carried out according to the Czechoslovak Pat. No. 127,660 and its corresponding British Pat. No. 944,307 checked at the temperature above the melting temperature of the polymer, e.g. at 250.degree. C and higher, and the equilibrium amount of the monomer is removed in vacuum. Expensive and complicated equipment is the main disadvantage of that procedure. The recovered caprolactam contains also undesirable cyclic oligomers and volatile products of side reactions and cannot be used again in the polymerization without purification. Other procedures, e.g. according to the Czechoslovak Pat. No. 113,971 checked corresponding to U.S. application No. 365,507, now abandoned and similarly according to the British Pat. No. 944,307, allow the reduction of the monomer content substantially if the polymerization is carried out at a temperature above or around the melting temperature of the polymer and the product is postpolymerized to equilibrium at a lower temperature.
High polymerization temperatures above the melting temperature of the polymer have some shortcomings. Side reactions which take place in any anionic polymerization are undesirable for the polymerization course as well as for the final product. Their extent is affected by the kind of catalyst components, their concentration and ratio, and also by the polymerization temperature. The activity time of the catalyst and its ability for postpolymerization are decreased at higher temperatures and the molecular weight is affected at the same time.